Conventionally, a spatial multiplexing transmission method that uses beamforming which is employed as an option for IEEE 802.11n is known as a method in which a single base station performs communication with multiple terminal stations. In this method, the base station transmits known signals that are mutually different in each beamforming within the transmission signal. Upon receiving these, by calculating a complex correlation value between the reception signal and the known signal in a known signal segment, each terminal station makes a transfer function estimation for each beamforming and for all of the antennas that are used by the terminal stations for receiving. Based on the results of these transfer function estimations, the signals that were spatially multiplexed on the transmitting side are demultiplexed on the receiving side.
The method used to demultiplex the signals in the terminal stations may be, for example, a Zero-Forcing (ZF) method in which complete interference rejection is performed, a Minimum-Mean-Squared-Error (MMSE) method in which the sum of the interference and the noise power on the receiving side is minimized, or a Maximum-Likelihood-Detection (MLD) method which is a maximum likelihood method (see, for example, Non-patent document 1 and Non-patent document 2).
Generally, when spatial multiplexing transmissions are used, there are frequently cases in which the number of spatially multiplexed signals being received by each terminal station is less than the number of reception antennas of each terminal station. At such times, by decreasing the number of antennas used in the processing of the reception signals, it is possible to greatly reduce the power consumption and the amount of signal processing in the receiving station. Here, in order to decrease the number of antennas used for reception in the terminal stations, it is necessary for the base station to select the antennas that are to be used for reception in a terminal station, and, in accordance with this selection, to perform beamforming such that the transmission signals to a particular terminal station are prevented from interfering with other base stations.    [Non-patent document 1 ] ‘802.11 High-speed Wireless LAN Textbook’, Morikura and Kubota, Impress R & D Publishing, April 2008    [Non-patent document 2] ‘Zero-Forcing Methods for Downlink Spatial Multiplexing in Multiuser MIMO Channels’, Q.H. Spencer, A.L. Swindlehurst, and M. Haardt, IEEE Trans. Sig. Processing, Vol. 52, No. 2, February 2004, pp. 461-471.